Which of these numbers is prime? ${2,\ 28,\ 35,\ 75,\ 85}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 2 are 1 and 2. The factors of 28 are 1, 2, 4, 7, 14, and 28. The factors of 35 are 1, 5, 7, and 35. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 85 are 1, 5, 17, and 85. Thus, 2 is a prime number.